Conventionally, an image forming apparatus transfers a toner image from an image carrier to a sheet, using a transfer member such as a transfer roller. As control of this transfer, constant voltage control and constant current control are known. In the constant voltage control, a problem such as transfer current shortage due to excessively large resistance of the transfer member, or abnormal discharge and so on due to excessively small resistance of the transfer member caused by environmental resistance variation of the transfer member may be generated. In addition, in the constant current control, transfer failure due to current shortage may be generated, by a leakage current from a place where the image carrier is in contact with the transfer member. In order to solve these problems, there is a system which applies a transfer voltage so that a proper transfer current is obtained, in accordance with a resistance value of the transfer member detected in a state in which a sheet is not present at a transfer position. In this system, the transfer voltage is corrected in accordance with the resistance value of the sheet which has been previously set for each kind of the sheet.
However, since the previously set resistance value (the setting resistance value) of the sheet is just an estimate value, a resistance value of an actual sheet can be greatly different from the setting resistance value. If the resistance value of the actual sheet is different from the setting resistance value, unmatching is generated between a transfer voltage to be set in accordance with the setting resistance value and a transfer voltage suitable for the resistance value of the actual sheet, in the transfer control. For the reason, there is a possibility that shortage of the transfer current or excess of the transfer current is generated to cause a defect. On the other hand, in a conventional image forming apparatus, it is necessary for a serviceman to manually perform setting of transfer control, and thereby it is difficult for a user oneself to change setting of a setting resistance value of a sheet and so on.